Invasive species: can they be stopped?

Britain is lagging behind world leaders in controlling invasive species. What can be done to hold back the marching organisms that destroy biodiversity and cause £1.7bn of economic damage each year? With your help, Karl Mathiesen investigates.


It's coming... An Asian hornet near a beehive in Fargues Saint Hilaire, south western France. The invasive insect has spread across France in just a few years and is expected to arrive in Great Britain in the near future, causing deep concern for beekeepers.
It's coming... An Asian hornet near a beehive in Fargues Saint Hilaire, south western France. The insect has spread across France in just a few years and is expected to arrive in Great Britain in the near future, causing concern for beekeepers. Honey bees have no immunity to the hornets and a hive can be quickly destroyed by the invaders. Photograph: Regis Duvignau/Reuters

My verdict

The answer to today's question is 'no' - with qualification. Britain's environment of free-trade, climate change and degraded local ecosystems has left it like Rome before Alaric paid a visit - ripe. Not all species will be repelled, probably not even most.

But if we ask, can some of them be stopped? The answer is a resounding yes. Citizen science, border control, education, regulation and international co-operation are all cost-effective ways to combat a problem that costs Europe €12bn every year. If these measures are not continued, improved and synchronised, this cost will only go up. It is encouraging to see the Environmental Audit Committee (EAC) undertaking their inquiry. British invasive species policy needs to be adapted to changing global circumstances.

A question the EAC will need to address is, how do we select species that are acceptable non-natives? And how do we triage invasive species that slip the net?

Money is one way to measure impact. Biodiversity is another. Invasive species create homogenous, dead ecosystems, dominated by single species. Or they threaten to wipe out whole species for which they are pathogenic - as with ash dieback and dutch elm disease. Human health and social impacts are also accounted for in the risk assessment process. 

The way in which these factors are balanced says a great deal about our values as a society. Cabi's Dick Shaw, told me this afternoon that once the risk assessment has been conducted, the decision on which species to spend money eradicating is ultimately a political one. I tried (without luck) to speak with the Non-Native Species Secretariat (NNSS) today. The NNSS board decides which species are targeted. Ensuring this process is transparent is important.

Part of this debate is informed by an (in my mind, misguided) aspiration to manage the world to fit an archetype we call 'nature'. This impulse idealises a moment in evolutionary time and seeks to freeze it, an ecosystem in a snow dome. The idea that nature stays put is deeply artificial. In this sense, conservation is not only unrealistic, it is a rejection of natural process. That's not to say we should open the floodgates. But eradicating a species on the basis that it is non-native exposes a conservatism that has little to do with science or the natural world.


Twitter reaction

Ghillie Dhu (@TheGhillieDhu)

Eradication is impossible for many of these invasive species as they are already established beyond hope

January 23, 2014

@brian_ebre @KarlMathiesen @guardianeco Damage caused by the apple snail in rice fields of the Ebro Delta (Catalonia)

January 23, 2014
ARKive (@ARKive)

Can you identify the signal crayfish? What about the Asian hornet? Test your invasive species knowledge with our quiz

January 19, 2014

It's important to draw a distinction between alien species (those which have been transported to a new area via human intervention) and alien invasive species (those alien species which are detrimental ecologically or economically), so in talking about invasive species we're only talking about the harmful species. Calling the likes of rabbits, horse chestnuts, etc, invasive is often used as a straw man in these discussions and it's frankly not helpful. In botany there's the concept of an archaeophyte, which is a species which is introduced, but so long ago (pre-1500CE) that modern-day effects are negligible.

The UK does now (thankfully) place a strict regulatory burden on the introduction of biocontrol species, largely because of the high-profile failure of some biocontrol species such as the Harlequin ladybird. The aim of the regulation is to screen out those species which are likely to have excessive non-target effects - eg, generalist predators which would eat the desired prey and then move on to something else. Introducing species is a numbers game - in general, around 1 in 10 species introduced will establish, and 1 in 10 of the established aliens go on to become invasive, so if you introduce enough species, eventually you'll get something harmful (harlequin, ruddy duck, grey squirrel - take your pick) to go with the largely-benign species (bryony ladybird in Surrey, European yellow scorpion in Sheerness, etc).

Ballast water

Along with land-based stowaways in containers, shipping's major contribution to the spread of invasive species has been through the dumping of ballast water. The International Maritime Organisation has developed a convention designed to limit the already vast impact of these species on marine environments across the world.



EAC reaction

Joan Walley MP. chair of the Environmental Audit Committee, who are conducting an enquiry into invasive species, said: 

"The Environmental Audit Committee is examining the Government’s approach to tackling invasive species at this critical juncture as negotiations continue in the European Commission on a new directive for a pan-European approach. That is important because species can spread quickly through international trade and the effects of climate change. We are at the beginning of our inquiry, taking our first evidence only yesterday, where we were pleased to hear that a previously proposed cap of 50 species to be targeted in the Directive seems to have been dropped — there are already nearly 2,000 invasive species in Britain alone! People and organisations can send in written submissions to our inquiry, using our website. ”

Risk assessment

Britain's primary scientific tool for making decisions about individual species is the risk assessment. The Great Britain Non-Native Species Secretariat says:

﻿Risk assessment is a key tool in the armoury against invasive non-native species.  It is a vital part of any comprehensive prevention strategy.

It can be used to aid prioritisation, to help enable effective rapid responses and for underpinning decision-making.  The GB Non-native Species Risk Analysis Mechanism was established in December 2006.  Within this mechanism risk assessments on non-native species are carried out by independent experts and these are reviewed by one peer reviewer and the risk analysis panel of experts (NNRAP).  Following this process risk assessments are available for comment before being finalised.

An example of the risk assessment on the Ring-necked parakeet, a commonly sighted UK pest, gives an indication of the primacy economic harm is given over biological impacts. Six questions address economic impact, whereas only two ask what biological impact the species will have.

Ring-necked parakeet.
Ring-necked parakeet. Photograph: Krys Bailey/Alamy


Citizen science

Helen Roy is an entomologist at the Centre for Ecology & Hydrology. She told the EAC yesterday:

"The UK is in the best position world-wide [to identify invasive species early] because of its so-called 'amateur' experts."

The great British tradition of backyard biologists may be the country's most important asset in the fight against invaders. The early identification of a species is often the difference between eradication and control. In terms of cost, early eradication is vastly cheaper.

Adrian Jowitt, from Natural England, says public education must be a priority for the UK government.

"We tend to concentrate on species that have arrived. One of the big things about this work is that it should be about risk management. If we stop these species before they arrive and if we can eradicate them before they establish then we can get rid of them for very little money. But we are not a society great at preventative measures. We are not educating. People are not alert to these issues."

Jowitt did note the most important area for education and regulation was not the general public, but the import and export industry, through which the majority of species arrive.

Below is a nice project that harnessed the power of Britain's citizen scientists:

Dan Parsons (@bedform)

Citizen science in action - tracking the effcet of invasive species on UK conker trees - #REF #Impact

January 23, 2014


As an example of innovative projects for invasive species detection, a new warning system is being developed to identify marine invaders in Welsh seas. The system involves a series of "settlement panels" which will be monitored by the fishing industry.

Gabrielle Wyn of Natural Resources Wales said, "The sooner we identify an invasive species - the more chance we have of successfully tackling it or removing it altogether. It will help put us on the front foot and will reduce the environmental and economic damage caused by these species. Working with the fishing and recreation industries is an important part of this project. They are best placed to spot these species early and reporting their finds will also benefit their businesses in the long term".
Dr Katherine Griffith, School of Ocean Sciences, Bangor University, said, "It is now widely recognised that invasive non-native species pose a significant threat to marine biodiversity and hence the functioning of coastal and oceanic ecosystems. (@NBTWT)

New warning system to find invasive species in Welsh seas

January 23, 2014


Strategies for stemming the tide

The Environmental Audit Committee (EAC) yesterday heard the situation was grim and in many cases the battle is already comprehensively lost. But there are strategies to avoid further invasion and to control existing ones. In September 2013, the European Commission released draft legislation for tackling the problem. Part of the EAC's inquiry will be to understand how Britain can best integrate with the new EU strategy. 

Black listing

According to the draft:

The proposal centres round a list of invasive alien species of Union concern, which will be drawn up with the Member States using risk assessments and scientific evidence. Selected species will be banned from the EU, meaning it will not be possible to import, buy, use, release or sell them. Special measures will be taken to deal with issues arising for traders, breeders or pet owners in the transitional period.


The proposal is for three types of intervention:

  • Prevention: Member States will organise checks to prevent the intentional introduction of species of concern. However many species come into the EU unintentionally, as a contaminant in goods or trapped in containers. Member States will have to take action to spot such pathways and take corrective measures.
  • Early warning and rapid response: when Member States detect a species of Union concern that is becoming established, they will take immediate action to eradicate it.
  • Management of established invasive alien species of concern: if species of Union concern are already widely spread, Member States will need to put in place measures to minimise the harm they cause.

One of the difficulties for Britain, and for the Europe-wide approach in general, is it will do little to stop intra-continental invaders, such as the killer shrimp from Eastern Europe. Dick Shaw deputy director of CABI told the EAC yesterday the UK must develop its own lists and responses which will integrate with the EU approach. Britain must also co-operate with other EU countries to ensure they are cognisant of the UK's specific danger species.


Your comments

Some excellent comments today!

Aha - finally one of my specialist subjects ! ...
You have talked about Asian hornets, grey squirrels, Rhododendrons, Japanese knotweed ... and others here. It is very dangerous to generalise, since there are such different biological issues involved (and note: emphasis on the need for scientific understanding).

A few observations:
- as long as we travel, trade and search for exotic garden plants and pets, invasive species are likely to 'crop up'.
- Australia and NZ are good examples to quote: they have had big problems to tackle for over a century...
- They have been enthusiastic adopters of classical biological control (BC): going to the origin of the invasive organism, and working out which natural enemies normally keep populations in check there. Where BC works well the benefit/cost can be spectacular.
- Although helping to implement BC in other countries, the UK places an enormous regulatory burden on implementing it here. Ask scientists at CABI what it has taken to implement measures against Jap. knotweed: the first authorisation here in over a century of the technique being proven.

... not that BC would necessarily solve the Asian hornet problem though - we would need to do the science. But then, the UK has been degrading it's applied biology for the whole of my career!

Moving away from the UK, I can give a bit of info on the situation on the river Ebro Delta, in Catalonia. Farmers and local ecosystems face huge dramatic consequences here from the Apple Snail invasion. I say "invasion" but all evidence seems to point to the original snails "escaping" from a small private firm which breeds exotic animals for aquariums. First question - how can a company of these characteristics be allowed to work inside a Natural Wildlife Park, Ramsar site, fragile wetlands etc? Although they've been taken to court it seems impossible that the firm, or govt members who signed permission, will pay the huge costs involved in trying to eliminate the snail. In 3 or 4 years they have colonised half the Ebro delta, leaving some farmers with a 25-50% loss of their rice crops. Obviously, if swift and determined action is not taken, it will be have environmental and economic consequences, probably leading to end of rice production here and all this involves (fresh water wetlands for birds, for example). And, I believe this is the only focal point in Europe - so obviously NOW is the time for the EU to act before it spreads. But, governments through a mix of incompetence and slow reactions have so far failed to prevent its spread.
There are other "invasive species" in the Ebro, with terrible effects on the river and its ecosystems. Zebra mussels, American crayfish etc - even a kind of huge cat fish that fishermen apparently brought from eastern Europe and released in the river for sport - now a big "tolerated" business, with many British fishermen coming especially for this fish despite it being a predator on native fish.

The crux of this issue boils down to political will. New Zealand has taken the threat of invasive species seriously and implemented strict national biosecurity measures to reduce the risk of new arrivals. The UK has shown no real political will to follow this example for fear of impacting trade or tourism. More than that, our trade agreements, particularly with the EU and the USA, make it very difficult for our government to actually ban the import of anything, and leave it open to legal challenges if it attempts to do so.
If the will is there and the right controls can be put in place, then it is not too late to 'turn the tide'. Even with climate change, the vast majority of new arrivals will be human mediated. This is a question of managing people, not nature.

If we are prepared to spend the money, then yes. The trouble is eradication schemes for invasives (like the grey squirrel for example) cost a lot and you need to keep on doing it for decades. An eradication scheme for Grey Squirrels, for example, would be a four decade long affair and all it takes is one government in that time span to cut funding and you are back to square one.

Invasive species is a thorny issue. Not all of them cause problems - would you want Little Owls wiped out for example? Some have been here for so long people think they are native - Corn Poppies? Rabbits? Some cause far less problems than people make out - Himalayan Balsam, for example. Then what about species which invade of their own accord through increasing their range - Collared Dove, White Egret, Tree Bumblebee?

Really, we are talking about harmful invasive species and we can't be sure it is harmful until it actually starts causing harm.

Having just finished a module on biological control, and in the process of doing my thesis on the ecology of invasions (focusing on gall wasps of oak) the prospect of complete eradication may be unrealistic but the focus of biological control isn't generally to eradicate something, but to keep populations at a manageable level.

As people here keep saying, "alien" does not equate to "problematic". Most species accidentally introduced here, such as scorpions, in crates of fruit, would never survive in the long term. Accidental introductions haven't been responsible for most of our most harmful invaders, but careless housing of farmed species and willingness to release something because it is deemed attractive (in the cases of mink and grey squirrels, respectively) as well as Japanese knotweed, which was prized as an ornamental until it became out of control, seem to have had an impact.

The wasps I've been looking at share almost the exact same niche with about 8 other native wasps and don't seem to be doing any harm. They've established themselves here from the continent of their own accord since we introduced their host, turkey oak.

We do have to accept that we will always have non-native species here, but I firmly believe that control- whether it be habitat management, introduced enemies or what have you- is possible in the majority of cases.


Green groups reaction

Woodland Trust head of conservation, Austin Brady, said today: 

“Our native wooded landscapes have never been more at risk to the threats posed by invasive species due to habitat fragmentation caused by poorly located development and inadequate expansion to protect existing woodland areas. The Woodland Trust believes the key is to try and build resilience, for example by ensuring our woodlands are full of a mixture of tree species of different ages and varieties. In the long term the landscape will then be better placed to recover if a pest or disease strikes."

Some measures the Woodland Trust is employing to prevent the spread of invasive species include:

Tree procurement:

To help ensure our wooded habitats are truly native the Trust is developing a tree procurement programme which will ensure all the trees it plants and provides to landowners and members of the public are native sourced and grown, leading the fight to tackle the spread of tree disease throughout the UK.


The Woodland Trust is involved in a LIFE+ funded partnership project to promote earlier detection of threats to tree health. We’ll be working with partners at National Trust, FERA, Forest Research and Forestry Commission to develop a tree health early warning system via an app, as well as recruiting and managing volunteers from across the UK who will help us detect and verify potential cases of tree disease.

ARKive (@ARKive)

@karlmathiesen take a look at our UK invasive species topic page for more information

January 23, 2014


Speed and scope of spread

Another key aspect of the invasive species dilemma is their ability to outcompete all other species and create imbalance in an ecosystem. Adrian Jowitt from Natural England says the prime examples in the UK are rhododendrons and killer shrimp, which have no natural predators and thrive unnaturally.

"They take over and become the dominant plant in the ecosystem," says Jowitt of rhododendrons. "They smother, for want of a better word, all other species."

As nicklpool says in the comment thread, the increase in the range of invasive species created by globalisation and trade causes many more organisms to appear in areas where they have no natural predators and can simply take over.

"The only historical difference is the speed at which species move from one place to another, something humans have greatly 'helped' with."

Hmm. Isn't the major difference that organisms spreading from Normany to Hampshire would face similar predators and so not run riot, by contrast with ones shipped in from Japan.

Individual invasive species can impoverish and simplify a local habitat just like a supermarket built on a nature reserve...

Natural History Museum botanist Mark Spencer, gave evidence to the EAC yesterday. He was also quoted on the Museum website making the case for urgent action and describing some of the worst offenders:

Dr Spencer said that the negative impacts on wildlife and human wellbeing of INNS (invasive non-native species) are becoming increasingly apparent.

For example, pollen from ragweed (Ambrosia artemisiifolia) is causing allergic reaction in people across Europe and in the UK, the tree of heaven (Ailanthus altissima), after 200 years of being non-invasive, has started to cause problems to the built environment.

Described as smelling like rancid cashew nuts, the tree of heaven is spreading alarmingly through the UK. Amateur gardeners are asked to be vigilant and remove seedlings.

It was originally considered a beautiful ornament, despite its name meaning 'foul-smelling tree' in Chinese.

Botanists have been warning for several years of its threat to plant health and biodiversity in the UK. Ailanthus leaks toxins into the ground that prevent the germination of other species in its vicinity. 

It also throws out underground suckers that damage pavements, drains and building foundations. 

According to Dr Spencer there have as yet been no significant attempts to manage tree of heaven in the UK. 

The tree of heaven is on Europe's '100 of the worst' list, which for the UK also includes the grey squirrel, Canada geese, Chinese mitten crabs, American mink, signal crayfish, rose-ringed parakeets, domestic goats and the Russian vine.

Some non-native species such as rainbow trout, sweet chestnut and horseradish have been introduced in the UK with no adverse effects. 

"INNS are a significantly increasing problem in Europe. The cost of managing them are estimated at £1.7 billion in the UK, and are expected to rise," Dr Spencer said.

What is an invasive species?

I suppose it depends on whether an encroaching species is considered invasive or whether it is in fact a natural reaction to climate change to replace one species with another which is better suited to the environment.

Difficult call if the encroachment happens naturally rather than by deliberate (or accidental) introduction.

PearOfAnguish has raised a great point in the comment thread about how we define an invasive species?There are over 2000 non-native species currently live in Britain, but only 200 of them are considered to be invasive.

The definition of an invasive species, as opposed to a non-native one, was presented to the Environmental Audit Committee hearing yesterday.

The International Union for the Conservation of Nature defines invasive species thus:


In our jaded, capitalist world however, the reality is that the British government and EU place economic impact alongside the impact on biodiversity. The Convention on Biological Diversity, which informs EU policy, defines invasive pests as:

The subset of alien species that are invasive can have significant environmental, economic and public health impacts and present a significant risk of the wholesale homogenisation of ecosystems.

There is little doubt that, in their risk assessments on invasive species scientists attempt to evaluate its economic impact. But one question I have is, how most weight is given to economy vs biodiversity? For example, if a species poses a risk to biodiversity, but little economic harm, is it taken as seriously as an animals like the Asian hornet, which presents a direct threat to the beekeeping industry?

There is a line of thought that natural migration should not be stifled and the acceptance of one species and not others is entirely subjective. Adrian Jowitt, principal adviser at Natural England, cites the example of sacred ibis as an invasive species that some tend to accept because it is large and charismatic. Ibis have begun to appear further north than ever (potentially due to climate change). Birding World describes the public response to its appearance in Europe and the potential damage it could cause to native bird species.

This addition to the European avifauna has been welcomed by some, due to the tameness and attractiveness of the birds, as well as the aura surrounding the species, which has been venerated since the time of the Pharaohs... Its feeding habits, however, are cause for real concern. In some areas, it is proving to be a serious predator of other bird species of conservation concern.

Jowitt says the response to invasive species must be based on scientific evidence and risk assessment.

Looking forward to the first comment arguing that trying to control Japanese Knotweed is tantamount to voting for the BNP (rabbits, Romans, etc.) Happens every time.

Protecting our environment against new arrivals is a matter for specialists in trade law and horticulture. In the meantime, rooting out the dreaded Knotweed, Balsam etc., is a very popular initiative ; maybe the various Wildlife Trusts could raise their profiles by organizing SWAT teams in welly boots...


Summary of the evidence session

The Environmental Audit Committee yesterday heard from a panel of experts. Some key points from the evidence were:

  • Invasive species, climate change and habitat loss were named as the three most significant threats to global biodiversity.
  • Climate change has made Britain more susceptible to new species proliferating where they did not before.
  • The degradation of Britain's natural environment means there is less natural resistance to the spread of dangerous organisms.
  • International trade is the major means of arrival for invasive species.
  • There is an increasing trend for species to come from much further afield than in the past.
  • Britain has a huge advantage over many other countries in Europe in combatting the spread of these species because it is an island.
  • There is a need to collaborate across the EU in order to stop the spread of these species. Although there is also a danger in focussing solely on pan-EU measures because some species are native to some parts of Europe and not to others.
  • The relative merits of black and white lists, which respectively ban or sanction the importation of certain species, were discussed and it was suggested that a combination of both could be used to regulate organism trade.
  • The need to prioritise the species which posed the greatest threat was outlined and MPs were informed that there is an adequate risk assessment process in place to make these decisions - although the panel indicated that it is often more cost effective to tackle a problem before waiting for a full study to take place.
  • Eradication is impossible for many of these species as they are already established beyond hope. The only approach then is to manage their encroachment on the native environment. This proves to be a far more costly exercise than an early eradication.
  • The UK is blessed with an outstanding tradition of amateur biologists who can act as a frontline detection force in catch new invaders early.

Welcome to the eco audit

Britain is more exposed than ever to non-native species that harm biodiversity and destroy industries. But the government spends just 1% of the annual damage bill on research into biosecurity measures. Experts say the UK is far behind world leaders, such as Australia and New Zealand, in tackling this problem.

Open borders, climate change, illegal trade and a degraded natural environment all contribute to creating porous borders and weakened native defences, meaning Britain is becoming susceptible to more and more invasive species.

The Environmental Audit Committee has launched an inquiry into Britain's policy in this area. Yesterday, Committee MPs heard evidence from a panel of scientists who emphasised the economic risk posed by the spread of non-native, damaging organisms. The British economy suffers losses of £1.7bn each year because of the damage they cause. Yet the government currently spends just £17m on research into protection. The panel outlined the methods of control available to the UK and the importance and difficulties in pan-EU co-operation. 

Today I'm asking for your help to discover what an invasive species actually is. Is it possible to turn the tide? Or is it now just a matter of triage? If so, how can the UK manage the proliferation of new organisms?
